In overview, what is Bitcoin Cash? How is it different from legacy bitcoin? What is the blockchain hard fork which was supposed to happen on August 1st, 2017? Who benefits from the fork? Who is disadvantaged? Why is Bitcoin Cash important? Why is the fork event important? Is this related to Segmented Witness (SegWit) and the debate on block size?

How will I know that the fork actually happened?

I am seeing questions and answers that assume this knowledge, but I don't see a good overview explanation here. I think bitcoin.stackexchange.com will benefit from having a good overview.

I ask as someone with general knowledge of cryptocurrencies like Bitcoin, and blockchain technology. But I have not followed the ins and outs of the Bitcoin community's debates recently.

If they have different values, can I arbitrage them?
– ValorumAug 2 '17 at 23:37

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@Valorum, comments are for clarifying and improving the current question, not for asking different questions. You have a good question. It might already be answered elsewhere on this stackexchange. If not, I encourage you to post it as a separate question.
– Jim DeLaHuntAug 2 '17 at 23:54

1 Answer
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The Bitcoin blockchain needed some protocol updates and there were several views on what those changes had to be. Because the blockchain is completely decentralized any part of the network can create and maintain its own version of the blockchain, that's called a hard fork, and that is what happened on August 1st.

On July 20th this year, the whole Bitcoin community voted at 97% in favor of implementing BIP 9 (Bitcoin Improvement Proposal) on August 1st, also known as SegWit. This basically changes the structure of the blocks so that the signatures header (the witness) are segregated from the actual transactions. From there it is possible to work out an increased block size. While the majority of the community was ready to implement an increased block size (SegWit2x) through a hard fork around November 2017, some big miners, willing to handle large blocks, decided to go ahead of SegWit2x and hard fork the chain, with blocks 8x bigger, right on August 1st hence creating Bitcoin Cash (BCH).

The fork happened (you can follow the forks here) and the first block was mined around 4PM (GMT) on August 1st. BCH starts trading on the platforms that agreed to also implement the increased block size (August 1st was a rough day for crypto-exchanges, some went down for a while). Because both chains share the same history, any owner of Bitcoin (BTC) technically also owns the same amount of BCH. As of now (August 3rd, 1AM - GMT), 16 blocks have been mined on the new chain, as the miners are waiting for the difficulty level to go down (the difficulty of mining a block on the chain depends on hash power, and the new chain inherited the old difficulty level but with only around 20% of total hash power, it'll take some time to adjust).

A hard fork is an important event because this is kind of an update of the whole network at the same time. Any big players could decide to do whatever they want as the blockchain is not some server that you can just update. This implies we can simply guess the outcome of a hard fork based on what information miners decide to disclose, but always stay aware that they can still change their minds.

The real issue here is that the current protocols running the blockchain are not scalable. For a real mass adoption of Bitcoin and other blockchain related technologies, we need to improve the protocols until we figure something stable and scalable. This might take a while and we will likely see other forks like this one with new ideas on how to scale the blockchain.

On to what will happen, this is complex and evolving everyday, but BCH could actually stay around as a bitcoin-equivalent crypto for a while, or just collapse in a few hours, or maybe BCH will survive as a low value crypto like any other Altcoin... really hard to tell. My view would be that one version of the chain will surpass the other one, and I find hard for the BCT to collapse anytime soon. Some might disagree, I must say I'm pretty new to the game (like a couple of weeks), but I put a lot of effort to try to understand what was happening. It is truly interesting to see how we, as a group, are trying to understand and find the ultimate implementation of one of the latest great technology upgrade that is the blockchain.

Best way to stay informed is the Bitcoin subreddit and for a more technical view on the markets the BitcoinMarkets.